You have multiplied the exponents 2 and 3 instead of adding them. Remember, when two numbers that are powers of the same base are to be muliplied, the product is a power of that same base. The exponent of the product will be the sum of the exponents of the two numbers that were multiplied.
In the present case the exponents (of 72 and 73) are 2 and 3. So, by the rule the exponent of the product will be 2 + 3. Now 2 + 3 isn’t 6, is it?